The Transmission Of Communicable Diseases Flashcards
Direct transmission
Here the pathogen is transferred directly from one individual to another by direct contact, inoculation and ingestion
Direct contact (contagious diseases)
-kissing or any contact with the body fluids of another person, eg bacterial meningitis and many sexually transmitted diseases.
-direct skin-to-skin contact eg ring worm or athletes foot
-microorganisms from faeces transmitted on the hands, for example, diarrhoeal diseases
Inoculation
-Through a break in the skin, eg during sex -HIV/AIDS
-from an animal bite eg rabies
-through a puncture wound or through sharing needles eg septicaemia
Ingestion
-taking in contaminated food or drink, or transferring pathogens to the mouth from the hands eg amoebic dysentery, diarrhoeal diseases.
Indirect transmission
Where pathogen travels from one individual to another indirectly, including formites, droplet infection and vectors
Formites
-inanimate objects such as bedding, socks or cosmetics can transfer pathogens eg athletes foot, gas gangrene.
Droplet infection (inhalation)
-Minute droplets of saliva and mucus are expelled from your mouth as you talk, cough or sneeze. If these droplets contain pathogens, when healthy individuals breathe the droplets in they may become infected eg influenza or tuberculosis
Vectors
-A vector transmits communicable pathogens from one host to another. Vectors are often but not always animals eg mosquitoes transmit malaria, rat fleas transmit bubonic plague, dogs, foxes and bats transmit rabies
-water can also act as a vector of disease eg diarrhoeal diseases.
Transmission between animals and humans
Some communicable diseases can be passed from animals to people, for example the bird flu strain H1N1 and brucellosis, which is passed from sheep to people. Minimising close contact with animals and washing hands thoroughly following any such contact can reduce infection rates. People can also act as vectors of some animal diseases, sometimes with fatal results, for example foot-and-mouth disease.
Factors which increase the probability of catching a disease
-overcrowded living and working conditions
-poor nutrition
-a compromised immune system, including in humans having HIV/AIDS or needing immunosuppressant drugs after transplant surgery
-In humans poor disposal of waste, providing breeding sites for vectors
-climate change-this can introduce new vectors and new diseases eg increased temperatures promote the spread of malaria as the vector mosquito species is able to survive over a wider area
-culture and infrastructure-in many countries traditional medical practices can increase transmission
-socioeconomic factors-for example, a lack of trained health workers and insufficient public warning when there is an outbreak of disease can also affect transmission rates.
Transmission of pathogens between plants
Plants don’t move around, cough or sneeze, yet diseases spread rapidly through plant communities, plant pollen and seed, for example move widely. Plants also have a less well developed immune system than humans.
Direct transmission
Involves direct contact of a healthy plant with any part of a diseased plant. Examples are ring rot, tobacco mosaic virus (TMV), tomato and potato blight, and black Sigatoka.
Indirect transmission- soil contamination
Infected plants often leave pathogens (bacteria or viruses) or reproductive spores from protoctista or fungi in the soil. These can infect the next crop. Examples are black Sigatoka spores, ring rot bacteria, spores of P.infestans and TMV. Some pathogens (often as spores) can survive the composting process so the infection cycle can be completed when contaminated compost is used.
Vectors-indirect transmission
-wind-bacteria,viruses and fungal or oomycete spores may be carried on the wind eg black Sigatoka blown between Caribbean islands, P.infestans sporangia form spores which are carried by the wind to other potato crops/tomato plants
-water-spores swim in the surface film of water on leaves;raindrop splashes carry pathogens and spores etc. Examples are spores of P. Infestans (potato blight) which swim over films of water on leaves.
-Animals-insects and birds carry pathogens and spores from one plant to another as they feed.Insects such as aphids inoculate pathogens directly into plant tissues.
-Humans-pathogens and spores are transmitted by hands, clothing, fomites, farming practices and by transporting plants and crops around the world. For example, TMV survives for years in tobacco products, ring rot survives on farm machinery, potato sacks etc.
Factors affecting the transmission of communicable diseases in plants
-Planting varieties of crops that are susceptible to disease.
-Over-crowding increases the likelihood of contact
-poor mineral nutrition reduces resistance of plants
-damp, warm conditions increase the survival and spread of pathogens and spores
-climate change-increased rainfall and wind promote the spread of diseases;changing conditions allow animal vectors to spread to new areas;drier conditions may reduce the spread of disease.
